Applied and Computational Mechanics 6 (2012) 93–106

Energy transformation and flow topology in an elbow draft tube

D. Stefana,∗, P. Rudolfa, A. Skotakb, L. Motycakb

aFaculty of Mechanical Engineering, Brno University of Technology, Technicka 2896/2, 616 69 Brno, Czech Republic

bCKD Blansko Engineering, a.s., Capkova 2357/5, 678 01 Blansko, Czech Republic

Received 20 January 2012; received in revised form 30 April 2012

Abstract

Paper presents a computational study of energy transformation in two geometrical configurations of Kaplan turbine

elbow draft tube. Pressure recovery, hydraulic efficiency and loss coefficient are evaluated for a series of flow

rates and swirl numbers corresponding to operating regimes of the turbine. These integral characteristics are then

correlated with local flow field properties identified by extraction of topological features. Main focus is to find the

reasons for hydraulic efficiency drop of the elbow draft tube.

c© 2012 University of West Bohemia. All rights reserved.

Keywords: draft tube, efficiency, pressure recovery, flow topology

1. Introduction

Draft tubes are diffusers placed at the outlet of the hydraulic turbine runner, see Fig. 1. Their

main purpose is transformation of the residual kinetic energy into pressure energy with maxi-

mum efficiency. Although geometrically relatively simple the internal flow can be quite com-

plex due to adverse pressure gradient, possible boundary layer separation, swirling and stream-

line curvature. The elbow draft tubes are usually used for a vertical arrangement of Kaplan

turbines. The most problematic part of the draft tube with negative influence on flow properties

is the elbow. Paper summarizes results of a computational study of energy transformation in

two geometrically different configurations of Kaplan turbine elbow draft tube.

Fig. 1. Longitudinal cross-section view of hydraulic power plant

∗Corresponding author. Tel.: +420 541 143 477, e-mail: y101274@stud.fme.vutbr.cz.

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

Fig. 2. Longitudinal cross-section view of DFT-A Fig. 3. Longitudinal cross-section view of DFT-B

Draft tube geometrical configuration A (DFT-A) – The elbow draft tube for vertical Kaplan

turbine includes one horizontal rib (in elbow part) and one vertical rib (in horizontal part), Fig. 2.

This draft tube is installed in hydro-power plant Strekov, Czech Republic.

Draft tube geometrical configuration B (DFT-B) – The elbow draft tube for vertical Kaplan

turbine without any ribs, Fig. 3. This draft tube is result of shape modification of DFT-A for

one specific velocity profile (different than velocity profile used in this work) [12].

Part of turbine runner hub (at the inlet of the draft tube), see Fig. 2 and Fig. 3, has been

included and modelled as a stationary wall for both configurations of the draft tube.

The numerical computation has been carried out in commercial CFD software ANSYS

FLUENT r.12. employing Reynolds Averaged Navier-Stokes equations to solve computational

domain by finite volume method. We used Realizable k − ε model of turbulence (RKE) with

non-equilibrium wall function to perform steady state solution. The RKE model of turbulence

and steady state of computation are chosen because of suitable capturing of flow properties and

computational demands for evaluation, where dynamics properties of flow are not extracted.

Each draft tube has been computed for a series of flow rates corresponding to operating regimes

of the turbine. The best efficiency point of the turbine (100 % QBEP ) corresponds to the mass

flow Qm = 440 kg/s. Inlet velocity profiles in Fig. 5 are the result of numerical computation of

the new design of Kaplan turbine runner for hydro-power plant Strekov and data were obtained

from CKD Blansko Engineering.

Investigation of the three-dimensional separation, carried out by software ANSYS CFD-

POST, and its influence on energy transformation is discussed. Skin friction lines, surface

streamlines and critical points are used to identify the topological features of the flow.

Fig. 4. Efficiency and hydraulic loss curves Fig. 5. Inlet velocity profile for CFD computation,

flow rate 100 % QBEP

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

2. Energy transformation

The hydraulic efficiency (1), pressure recovery factor (2) and hydraulic loss coefficient (3) have

been evaluated in several cross-sections for series of flow rates and swirl numbers

η =ps(2) − ps(1)

pd(1) − pd(2), (1)

cp =ps(2) − ps(1)

12ρv2(1)

, (2)

ξ =2

v2(2)

[(

α(1)v2(1) − α(2)v

2(2)

2

)

+ps(1) − ps(2)

ρ

]

, (3)

where ps is static pressure, pd is dynamic pressure, v is mean velocity and α is Coriolis number.

The number in parentheses represents 1 = inlet and 2 = outlet. The global development of these

integral characteristics (1–3) is plotted in Fig. 6 for DFT-A and in Fig. 7 for DFT-B.

2.1. Energy transformation in DFT-A

Changes of DFT-A cross-sectional area are nearly linear from inlet to outlet of the draft tube.

For flow rates 95.5 % QBEP and 100 % QBEP decrease of pressure recovery factor at the elbow

part is observed and as will be shown in section 2.1, it is caused by flow separation under

horizontal rib. For 109.1 % QBEP , the flow separation is significantly reduced, therefore, any

considerable decrease of pressure recovery factor is not apparent.

The other decrease in pressure recovery factor is evident between cross-sections 9 and 11

for all three flow rates and is caused by the beginning of the vertical rib. The volume of vertical

rib reduces cross-sectional area and thereby increases velocity and frictional losses. This effect

is a consequence of the design so therefore it occurs for every flow rate. The smallest back-flow

region at horizontal part of draft tube for whole range of flow rates is observed for 109.1 %

QBEP . This is the reason for steeper slope of the pressure recovery curve than for other oper-

ating points. The highest value of DFT-A efficiency (1) is derived from efficiency curve plotted

in Fig. 4 and corresponds to flow rate 103 % QBEP .

2.2. Energy transformation in DFT-B

The shape modification of DFT-B has had appreciable influence on the development of cross-

sectional area at the elbow part of draft tube, see area between cross-section 3 and 6 in Fig. 7.

The flow cross-section at the end of elbow part has been reduced because of separation risk

on the inner curved wall. This modification was done for some past design of the draft tube

with slightly different inlet velocity profile. It was anticipated that it might be beneficial also in

combination with new runner design, which has to some degree altered outlet blade angles and

hence also the outlet velocity components. Unfortunately, this assumption was not confirmed,

flow inside the draft tube is rather sensitive to inlet boundary conditions and no positive impact

for the DFT-B modification was observed.

DFT-B draft tube features rather steep increase of pressure recovery factor at the inlet part,

which is connected with large wall divergence leading into reduction of velocity and abrupt

transformation of kinetic energy into pressure energy. This is an undesirable effect, because too

low kinetic energy of flow stream brings risk of flow separation at the downstream parts of the

draft tube. In Fig. 13, it is shown that a large part of kinetic energy is dissipated just at the inlet

part of draft tube and as will be shown in section 6.1, the flow separation occurs at the elbow

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

part of the draft tube. The highest value of DFT-B efficiency (1) is derived from efficiency curve

plotted in Fig. 4 and corresponds to flow rate 92 % QBEP .

3. Back-flow regions

The back-flow regions block flow area, increase velocity and cause higher hydraulic losses.

DFT-A: The first significant back-flow region is situated under the horizontal rib and oc-

cupies right (for flow rate lower than that of the best efficiency point of turbine, Fig. 8) or left

(for flow rate higher than that of the best efficiency point of turbine, Fig. 9) side of the draft

tube. The highest suppression of this region is reached for flow rate 109.1 % QBEP , Fig. 10.

The second back-flow region is situated on the top of horizontal part of the draft tube and is

the most suppressed between operating points 104.5 % QBEP and 109.1 % QBEP . The highest

hydraulic efficiency of DFT-A is reached for 103 % QBEP when the back flow regions are not

the smallest. It shows influence of higher hydraulic losses caused by surface friction on the ribs

when the flow rate increases.

DFT-B: Two back-flow regions occur. The first one is situated at the end of elbow part

and second one at the horizontal part of draft tube. For flow rate 104.5 % QBEP , they are the

largest and connected together, as shown in Fig. 10. The largest suppression of these regions is

observed close to flow rate 95.5 % QBEP , Fig. 11.

4. Contours of dissipation

The dissipation function D for computation of turbulent flow by RANS equation is defined as

follows

D = 2

∫ ∫

V

∫

{

(µ+ µt)

⌈

(

∂vx

∂x

)2

+1

2

(

∂vx

∂y+

∂vy

∂x

)2

+1

2

(

∂vx

∂z+

∂vz

∂x

)2

+

(

∂vy

∂y

)2

+1

2

(

∂vy

∂z+

∂vz

∂y

)2

+

(

∂vz

∂z

)2⌉}

dV . (4)

For both draft tubes, the contours of dissipation (4) were computed at several cross-sections.

Areas with the highest value of dissipation are coloured from black to white/vanish (over-range

of colormap). The range of the colormap has been set and reduced so that only the dissipation

inside of the volume is visible, because the highest value of dissipation occurs in boundary layer

regions.

DFT-A: Several important areas with high value of dissipation are observed. The first one

is at the inlet part of the draft tube. Considerable dissipation is caused by high velocity gradient

of stream coming out from turbine runner. The second area is under the horizontal rib and

corresponds with the back-flow region, compare Fig. 12 and Fig. 8. This result confirms that

the back-flow regions are highly dissipative. The third area is located behind the trailing edge

of the horizontal rib and is caused by sweeping of boundary layer into the main stream.

DFT-B: The main area of high dissipation is located close to the inner bend radius, Fig. 13.

It is caused by very high velocity gradients induced by flow inside the elbow part. As in the

case of DFT-A, significant dissipation caused by stream coming out from the turbine runner is

observed at the inlet part of the draft tube.

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

Fig. 6. Hydraulic efficiency, pressure recovery factor and loss coefficient evaluated in several cross-

sections for three flow rates 95.5 % QBEP , 100 % QBEP and 109.1 % QBEP in case of DFT-A

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

Fig. 7. Hydraulic efficiency, pressure recovery factor and loss coefficient evaluated in several cross-

sections for three flow rates 95.5 % QBEP , 100 % QBEP and 109.1 % QBEP in case of DFT-B

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

Fig. 8. Back-flow regions (in grey colour) in case

of DFT-A, flow rate 104.5 % QBEP

Fig. 9. Back-flow regions (in grey colour) in case

of DFT-A, flow rate 109.1 % QBEP

Fig. 10. Back-flow regions in case of DFT-B flow

rate 104.5 % QBEP

Fig. 11. Back-flow regions in case of DFT-B flow

rate 95.5 % QBEP

Fig. 12. Contours of dissipation in case of DFT-A Fig. 13. Contours of dissipation in case of DFT-B

5. Topology of the three-dimensional separation

As mentioned in Tobak and Peak [13] and also in Depardon et al. [2], the hypothesis proposed by

Legendre in 1956 brought a mathematical framework for description of the three-dimensional

flow separation. The hypothesis is based on shear-stress patterns and critical points situated in

the flow field and corresponding with the three-dimensional separated flow.

Fig. 14. Skin friction lines and surface streamlines

in case of DFT-A

Fig. 15. Skin friction lines and surface streamlines

in case of DFT-B

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Fig. 16. Types of critical points

The three types of critical points in the flow field are identified, see Fig. 16. Focus F

is a point where the vortex filament of the three-dimensional separation core is concentrated.

Surface friction lines go into (stable) or out (unstable) of the centre of focus. Saddle point S is

a place where surface friction lines converge from one side and diverge to the other side. Node

N: Surface friction lines go into (stable) or out (unstable) of the centre of node. Node usually

lies near or directly onto solid surfaces.

For the exact determination of critical points in particular flow cross-section computation of

velocity gradient tensor eigenvalue (5) is used [3, 4]

vij |xi,s=

[

∂vx∂x

∂vx∂y

∂vy∂x

∂vy∂y

]

. (5)

Each critical point (saddle, stable and unstable node, stable and unstable focus) is defined by

sign of the eigenvalue of the tensor (5).

In case of elbow draft tube, several regions with risk of the three-dimensional separation

occur. The beginning of the three-dimensional flow separation lies onto draft tube body surface

where boundary layer separates and is carried out by main stream. The vortex region originates

from this separation and causes blockage effect leading to the flow acceleration.

Exact visualization and evolution of separation core (so-called dividing surface) in flow field

is rather difficult, especially when dealing with 3D data. One of the possibilities is to use the

Sujudi-Haimes algorithm [9]. The computational algorithm looks for the points in the velocity

field where a single real eigenvector exists and this is parallel to the velocity vector [5, 6].

There are also other approaches to visualize separation surfaces emanating from critical points,

see [7, 10]. Some of them are specifically focused on swirling flows, see [1].

In this work, prediction of the core evolution is solved only by visualization method based

on searching of foci in particular draft tube cross-sections (surface streamlines) and on the solid

surfaces (skin friction lines), Fig. 14 and Fig. 15. This method is simplification of finding

possible occurrence of core but not very suitable for tracking the spatial evolution of the core.

Setting the cross-section orientation represents a very difficult task, because foci are properly

visible only on cross-section which is almost exactly perpendicular to the separation core. It is

also advised to observe back-flow regions, which are related to global flow separation and lead

to the hydraulic efficiency drop.

5.1. Global flow separation

In case of elbow diffusers, the main part with risk of the three-dimensional separation is the

bend. For rectangular curved diffuser, the authors of [8] investigated three kinds of global flow

separations: massive, typical and simple. The typical idea of global flow separation: “The

global flow separation begins, where the flow is separated from the wall and formed the back-

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Fig. 17. Right view on elbow part of DFT-A for

flow rate 95.5 % QBEP

Fig. 18. Right view on elbow part of DFT-A for

flow rate 100 % QBEP

Fig. 19. Right view on elbow part of DFT-A for

flow rate 104.5 % QBEP

Fig. 20. Left view on elbow part of DFT-A for flow

rate 109.1 % QBEP

flow regions” is stated in [8]. The global flow separation is highly dissipative phenomenon,

increases hydraulic losses and reduces hydraulic efficiency.

In case of turbulent flow in the draft tube (unsteady, three-dimensional with rotational char-

acter), the flow topology of the separation is very complex and directly corresponds with shape

of the draft tube. This statement is especially characteristic of the draft tube containing ribs.

Types of the three-dimensional separation corresponding to each of the investigated draft tubes

will be shown in sections 5 and 6.

5.2. Local flow separation

In contrary to the case of global flow separation, the local flow separation is not related to back-

flow regions. Hence no significant negative effect, as lower efficiency due to higher dissipation,

has been documented. The example of local type of three-dimensional separation is observed

in section 5.2 for the DFT-A draft tube.

6. Three-dimensional separation in DFT-A draft tube

6.1. Separation under horizontal rib

This separation is mainly caused by leading edge of horizontal rib which suppresses stream ro-

tation at the inlet of the draft tube. This separation is in combination with back-flow region that

means it is the global flow separation which decreases energy transformation and deteriorates

efficiency.

Flow rate: (Figs. 17–20)

• 95.5 % QBEP : The back-flow region is very large and starts from leading edge of the

horizontal rib, see Fig. 17, and develops downstream to the draft tube. The global flow

separation is represented by saddle point S in combination with focus F .

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

Fig. 21. Right view on horizontal part of DFT-A

for flow rate 95.5 % QBEP

Fig. 22. Right view on horizontal part of DFT-A

for flow rate 100 % QBEP

Fig. 23. Right view on horizontal part of DFT-A

for flow rate 104.5 % QBEP

Fig. 24. Right view on horizontal part of DFT-A

for flow rate 109.1 % QBEP

• 100 % QBEP : The back-flow region and global flow separation are smaller by getting

closer to the best efficiency point of the draft tube, see Fig. 18.

• 104.5 % QBEP : This flow rate is close to the best efficiency point. The global flow

separation is considerably reduced but still observed, see Fig. 19.

6.2. Separation at horizontal part of draft tube

The global and the local types of flow separation occur at the horizontal part of draft tube.

The global flow separation with large back-flow region is observed for higher and lower flow

rates out from the best efficiency point. The local flow separation is caused by boundary layers

sweeping from trailing edge of horizontal rib. These boundary layers are then carried out by

main stream and formed into separation core.

Flow rate: (Figs. 21–24)

• 95.5 % QBEP : The large back-flow area surrounds the top of vertical rib. The global

flow separation starts in Fp onto roof of draft tube body and winds into main stream. The

local flow separation is represented by node N and focus F , Fig. 21.

• 100 % QBEP : The separation with beginning in Fp is smaller than previous one but still

considerable back-flow region occurs. Separation core in Fig. 22 is wound through the

focus F1. Node N and focus F2 represent the local flow separation.

• 104.5 % QBEP : This flow rate is close to the best efficiency point of the draft tube. The

global flow separation in Fig. 23 is almost reduced. On the other hand, the local flow

separation with core in focus F is getting stronger.

• 109.1 % QBEP : High extensive local flow separation with two cores goes through the

foci F2 and F3 as seen in Fig. 24. The small global flow separation with beginning in Fp

and separation core going through F1 is located near the top of vertical rib.

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

Fig. 25. Top view on DFT-B, flow rate 95.5 %

QBEP

Fig. 26. Top view on DFT-B, flow rate 100 %

QBEP

Fig. 27. Top view on DFT-B, flow rate 104.5 %

QBEP

Fig. 28. Detailed view on global flow separation

beginning, flow rate 104.5 % QBEP

Fig. 29. Top view on DFT-B, flow rate 109.1 %

QBEP

Fig. 30. Top view on DFT-B, flow rate 113.6 %

QBEP

Only the right channel of horizontal part is shown for demonstration. The same kinds of the

three-dimensional separations occur also in the left channel.

7. Three-dimensional separation in DFT-B draft tube

The main region of elbow draft tube (without ribs) where could be formed the three-dimensional

separated flow is at the inner radius of the bend.

7.1. Separation at the end of elbow part of draft tube

Flow rate: (Figs. 25–30)

• 95.5 % QBEP : The unstable node N in combination with saddle S forms the beginning

of separation. The back-flow region is very small and is situated far away from separation

beginning. Hence this separation should be local type of separation, see Fig. 25.

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

• 100 % QBEP : The unstable node N in combination with saddle S forms the beginning of

global flow separation because the small back-flow region is formed at the end of elbow

part, see Fig. 26.

• 104.5 % QBEP : The beginning of large global flow separation is represented by combina-

tion of saddle Sp with foci Fp1 and Fp2 on solid surface. From these foci separation cores

start, which go through foci F1 and F2 situated in flow cross sections 2–7, see Fig. 27 and

Fig. 28.

• 109.1 % QBEP : The global flow separation is still extensive, but diminishes and only one

focus Fp on draft tube body occurs in Fig. 29.

• 113.6 % QBEP : The flow topology in Fig. 30 is similar to the flow rate 100 % QBEP as

shown in Fig. 26. At the end of elbow part is formed the small back-flow region which

leads to global flow separation.

The detailed view on the beginning of global-flow separation for flow rate 104.5 % QBEP is

shown in Fig. 28. The two separation cores start in foci Fp1 and Fp2, which are in combination

with saddle Sp. Nodes N1 and N2 and saddle S characterize the separation in flow cross-section.

8. Swirling flow

The swirling flow at the draft tube inlet is mainly caused by tangential velocity of stream exiting

from the turbine runner. The swirl number Sn [11] is used for description of the swirling flow

Sn =

∫

vaxvtanr dS

R∫

v2ax dS, (6)

where vax is axial velocity, vtan is tangential velocity, r is radial coordinate and R is inlet radius

of the draft tube. The swirl numbers computed by (6) in the inlet cross-section are plotted in

Fig. 31. It can be seen that from flow rate 104.5 % QBEP to higher the flow rotates in op-

posite direction than turbine runner. Fig. 32 shows hydraulic efficiency curves for each draft

tube against values of the swirl numbers. The best efficiency point for DFT-A is obtained for

lower swirl number than in the case of DFT-B. This difference shows the significant influence

of horizontal guide rib on flow in draft tube in relation to global flow separation caused by this

rib. Result for DFT-A indicate that low swirling flow at the inlet of the draft tube is important

Fig. 31. Swirl number Sn versus flow rate Qm Fig. 32. Draft tube efficiency η vs. swirl number Sn

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

for smoother splitting of water stream by leading edge of the horizontal rib. Guide rib acts like

inhibition for swirling flow exiting from the runner. Thus, for the highest or lowest value of the

swirl number, the back-flow region situated under horizontal rib and three-dimensional separa-

tion grows as well. On the other hand, the higher swirling flow with positive value of swirl num-

ber (the swirl rotates in same direction as turbine runner) is required for flow rate corresponding

with the best efficiency point of DFT-B. Higher swirling flow pushes the boundary layer against

the walls, which reduces the risk of separation mainly in the elbow part of draft tube.

9. Conclusion

The main result of this work is referring to the global flow separation influence on the draft tube

efficiency. The global flow separation in combination with the back-flow regions is a highly

dissipative phenomenon. When flow rate moves out of the best efficiency point of the draft

tube, then extension of the global flow separation is observed for both draft tubes. Insignificant

role on draft tube efficiency is also hydraulic losses caused by surface friction, mainly in case

of DFT-A with two ribs.

The efficiency maximum of DFT-A is obtained for flow rate 103 % QBEP and swirl number

Sn = 0.05. The global flow separation is the most considerably reduced between flow rates

104.5 % QBEP and 109.1 % QBEP and for swirl number close to Sn = 0. These results show

compromise between frictional losses which grow with increasing flow rate, and non-swirling

flow ensuring the lowest three-dimensional separation. But this relation is only valid for case

of DFT-A or the similar elbow draft tubes with guide rib.

In case of DFT-B the swirl flow does not have undesirable effect, on the contrary the swirling

flow with Sn = 0.25 in the best efficiency point of draft tube prevents risk of separation. It is

seen that for flow rate 104.5 % QBEP and Sn = 0 the largest global flow separation is reached

with important influence on the draft tube efficiency. This separation is in large amount caused

by non-swirling flow.

It is difficult to describe exact character of the local type of flow separation in draft tubes

because it does not correspond with back-flow regions. Thus, it becomes difficult to predict

evolution of separation core. Present investigation indicates that local type of flow separation

has no evident influence on the draft tube efficiency.

The results of the flow topology confirm that the patterns of skin friction lines onto solid

surfaces are good indicators for quality of flow in particular region of the flow field. Identifi-

cation of the skin friction patterns is rather sensitive tool to observe flow variations in the draft

tube interior, if the inlet velocity profile is changed. In other words skin friction lines enable

convenient observation how design modifications are reflected in flow topology. Combining the

information about details of the flow field with integral parameters like hydraulic efficiency or

loss coefficient, it offers full picture enabling optimization of the draft tube shape or the runner

blade outlet angles.

Acknowledgements

Authors gratefully acknowledge support of the research by Czech Science Foundation under

project No. 101/09/1715 “Cavitating vortical structures induced by rotation of liquid” and by

Ministry of Trade and Industry under project No. 2A-1TP1/108 “Performance increase and

operational range broadening for upgraded low head hydraulic power plants”.

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D. Stefan et al. / Applied and Computational Mechanics 6 (2012) 93–106

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